Explosive-detonation and autoignition of end gas in a combustion chamber produce a metallic sound, commonly referred to as knock. Knock is caused by improper ignition of fuel in an internal combustion engine. Improper ignition results in decreased engine performance and increased emissions. Knock, furthermore, generates acoustic vibrations which propagate throughout the engine structure, and possibly other adjoining structures. These vibrations, coupled with a rapid rate of pressure rise in the combustion chamber, may promote accelerated wearing of engine components. Wear may be even faster for engines operating with natural gas, due to a higher rate of pressure rise in the combustion chamber as compared with gasoline powered engines.
Detonation in an engine may be sensed by either a pressure sensor or a vibration sensor. Pressure sensing will provide only a knock signal; however, it is very costly where pressures are high, as in a diesel engine. Vibration sensing to measure the magnitude of detonation in an engine is difficult. Noise and vibrations unrelated to knock contaminate vibration sensor signals. Contaminated signals require complex filtering in order to detect signal components related to detonation. Alternatively, expensive sensors that may provide cleaner signals requiring less complex filtering. However sensor outputs fluctuate, thus inhibiting precise measurement of knock. For example, even supposedly identical accelerometers often vary due to manufacturing techniques, and as operating conditions change, the accelerometers deliver inaccurate signals.
Prior systems provide means for detecting knock and controlling selected engine operating parameters to reduce the knock to an acceptable level. Recently these efforts have been directed to sensing knock induced vibrations by monitoring one or more characteristic frequencies corresponding to the acoustic cavity resonance modes of the combustion chamber. These characteristic frequencies generally act as carrier waves and are amplitude modulated by the knock level vibrations. When demodulated, the magnitude of the envelope of the carrier wave denotes the magnitude of the knock. Discovering and monitoring cavity resonance frequencies reduces the amount of filtering needed to obtain acceptable detonation information. This information is typically used to reduce knock by retarding the spark advance. For example, U.S. Pat. No. #4,364,260 issued to Chen et al. on Dec. 21, 1982 discloses a knock detecting apparatus having an accelerometer tuned to the acoustic cavity resonance frequencies of the engine cylinders.
Air/fuel ratio controls and spark retarders attempt to reduce knock to acceptable levels, but these controls and their sensors can malfunction. Known oxygen sensors, in particular, have relatively short lives of 1000 to 2000 hours. In automotive applications this life expectancy is acceptable. However, work engines may be required to perform in excess of 10,000 hours. Should a sensor or control fail, the engine would be susceptible to possibly damaging knock. Furthermore, retarding has limits, since the engine will not operate if timing is severely retarded.
The present invention is directed to overcoming one or more of the problems as set forth above.